Magnetic random access memory and method for manufacturing the same

ABSTRACT

A magnetic random access memory is provided including a substrate, a magnetoresistance element which includes a ferromagnetic layer having an invertible spontaneous magnetization, which varies in resistance according to the direction of the spontaneous magnetization, and is formed above the substrate, and a wiring which extends in a first direction and is used for making an electric current flow to generate a magnetic field to be applied to the magnetoresistance element. The wiring is formed so as to pass through a first position which is closer to the substrate than the magnetoresistance element and does not overlap the magnetoresistance element when viewed from a direction perpendicular to the main surface of the substrate, and a second position being above said magnetoresistance element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic random access memory(hereinafter, referred to as “MRAM”). The present invention relatesparticularly to a technique for making it possible to write data into amemory cell of MRAM by means of a smaller write current.

2. Description of the Related Art

An MRAM has become a key device as a nonvolatile memory which can bewritten at a high speed and can be rewritten a large number of times.

As shown in FIG. 1, a typical memory cell of MRAM comprises amagnetoresistance element composed of a pin layer 101 having a fixedspontaneous magnetization, a free layer 102 having an invertiblespontaneous magnetization and a non-magnetic spacer layer 103 providedbetween the pin layer 101 and the free layer 102. The free layer 102 isformed so that the direction of its spontaneous magnetization is allowedto be parallel or anti-parallel with the direction of spontaneousmagnetization of the pin layer 101.

The memory cell stores data of one bit as the direction of spontaneousmagnetization of the free layer 102. The memory cell can take two statesincluding a “parallel” state where the spontaneous magnetization of thefree layer 102 and the spontaneous magnetization of the pin layer 101are parallel with each other and an “anti-parallel” state where thespontaneous

magnetization of the free layer 102 and the spontaneous magnetization ofthe pin layer 101 are anti-parallel with each other. The memory cellstores data of one bit by making one of the “parallel” state and the“anti-parallel” state correspond to “0” and making the other correspondto “1”.

A read operation of data from the memory cell is performed by detectingthe change in resistance of the memory cell caused by amagnetoresistance effect. The directions of spontaneous magnetization inthe pin layer 101 and the free layer 102 have an influence on theresistance of a memory cell. In case that the directions of spontaneousmagnetization of the pin layer 101 and the free layer 102 are parallelwith each other, the resistance of the memory cell has a first value R,and in case that they are anti-parallel with each other, the resistanceof the memory cell has a second value “R+ΔR”. The directions ofspontaneous magnetization of the pin layer 101 and the free layer 102enable data stored in a memory cell to be detected by the resistance ofthe memory cell.

A write operation of data into a memory cell is performed by making awrite current flow in a word line and a bit line provided in a memorycell array and inverting the direction of spontaneous magnetization ofthe free layer 102 by means of a magnetic field generated by said writecurrent.

The reduction of a write current necessary for writing data is importantfrom the viewpoint of reducing the power consumption of MRAM. Atechnique of reducing a write current has been disclosed in Japanesepublished application 2002-110938A. In this application, a magneticfield is concentrated at a magnetoresistance element by joining ahigh-saturation magnetization soft magnetic material or a metal-nonmetalnano-granular film to a signal line in which a write current is made toflow, and thereby a write current is reduced.

Another MRAM having a structure for reducing a write current has beendisclosed in U.S. Pat. No. 5,732,016. In the MRAM disclosed in thispatent, a coil is used as a wiring in which a write current is made toflow, and a magnetoresistance element is inserted in the coil. Since amagnetic field to be applied to the magnetoresistance element is inproportion to the number of turns of the coil, a write operation can beperformed by means of a smaller write current.

Other MRAMs having a structure for reducing a write current have beendisclosed in U.S. Pat. No. 6,236,590 and Japanese Patent application2002-118239. In the MRAM disclosed in U.S. Pat. No. 6,236,590, the widthof a conductor in which a write current is made to flow is made smallerthan the width of the data storage layer. By making small the width of aconductor in which a write current is made to flow, the misalignmentbetween the conductor and the data storage layer is prevented and theleakage of a magnetic field generated by a write current is reduced andtherefore a write operation can be performed by means of a smaller writecurrent.

Furthermore, another technique has been disclosed in Japanese publishedapplication 2000-82283A. In a magnetic storage device disclosed in2000-82283A, a structure having a coupling control layer disposedbetween two magnetic layers is used. One of the two magnetic layers isused as a storage carrier. In the both magnetic layers, a driving lineis provided in a direction parallel with the direction of spontaneousmagnetization possessed by the magnetic layers. In case that a writeoperation of data into the storage carrier is performed, an electriccurrent is made to flow in the driving line and a magnetic field isapplied in a direction perpendicular to the direction of spontaneousmagnetization possessed by the magnetic layer used as a storage carrier.The inversion of the spontaneous magnetization possessed by the magneticlayer used as a carrier is made selectively easy. The inversion of thespontaneous magnetization possessed by the magnetic layer used as astorage carrier is performed by an exchange interaction acting on thetwo magnetic layers through the coupling control layer. The driving linein which an electric current is made to flow at the time of a writeoperation of data is formed so as to be curved upward.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an MRAM memory formaking it possible to reduce write current for a write operation of datainto an MRAM memory cell.

According to a first aspect of the present invention, a magnetic randomaccess memory comprises a substrate, a magnetoresistance element whichincludes a ferromagnetic layer having an invertible spontaneousmagnetization, which varies in resistance according to the direction ofthe spontaneous magnetization, and is formed above the substrate, and awiring which extends in a first direction and is used for making anelectric current flow to generate a magnetic field to be applied to themagnetoresistance element, wherein the wiring is formed so as to passthrough a first position which is closer to said substrate than themagnetoresistance element and does not overlap the magnetoresistanceelement in case of being viewed from a direction perpendicular to themain surface of said substrate, and a second position being above saidmagnetoresistance element.

In the first aspect of the present invention, according to such astructure of the wiring, an electric current made to flow in the wiringhas a horizontal current component flowing in a horizontal directionrelative to the main surface of the substrate and a vertical currentcomponent flowing in a direction perpendicular to the main surface inthe vicinity of the magnetoresistance element. A magnetic fieldgenerated by the horizontal current component and a magnetic fieldgenerated by the vertical current component are added to each other inthe magnetoresistance element since they coincide in direction with eachother, and the resultant large magnetic field is applied to themagnetoresistance element. Due to this, a write operation of data can beperformed by means of a smaller write current.

According to a second aspect of the present invention, a magnetic randomaccess memory comprises a substrate, a ferromagnetic layer having aninvertible spontaneous magnetization, and formed above the main surfaceside of the substrate, a first wiring which extends in a first directionsubstantially parallel with the substrate and for flowing an electriccurrent to invert the spontaneous magnetization wherein theferromagnetic layer is substantially symmetrical with respect to a planeof symmetry being substantially perpendicular to the substrate, whereina centerline of the first wiring is shifted to the plane of symmetry.

According to the second aspect of the present invention, the firstwiring for flowing an electric current to invert the spontaneousmagnetization generates the largest write magnetic field in the vicinityof the center line of the wiring. The center line of the first wiringbeing a position where the largest write magnetic field is generated ismoved toward the outer edge of the free ferromagnetic layer by arrangingthe first center line of the first wiring to be offset from the firstplane of symmetry of the free ferromagnetic layer. When the first centerline is moved close to the outer edge of the free ferromagnetic layer, awrite current necessary for inverting a domain in the outer edge of thefree ferromagnetic layer is made smaller. When a domain in the outeredge of the free ferromagnetic layer is inverted, the inversion ofdomain propagates from the outer edge of the free ferromagnetic layer tothe center and the spontaneous magnetization of the free ferromagneticlayer is completely inverted. Therefore, by arranging the first centerline of the first wiring so as to be offset from the first plane ofsymmetry of the free ferromagnetic layer, it is possible to make small awrite current necessary for inverting the spontaneous magnetization ofthe free ferromagnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 (a) and (b) are the conventional magnetic random access memory.

FIG. 2 is a sectional view showing a first embodiment of a magneticrandom access memory according to the present invention.

FIG. 3 is a top view showing the first embodiment of a magnetic randomaccess memory according to the present invention.

FIG. 4 is a sectional view showing the structure of a magnetoresistanceelement 5.

FIG. 5 is a sectional view showing a method for manufacturing a magneticrandom access memory of the first embodiment.

FIG. 6 is a sectional view showing a method for manufacturing a magneticrandom access memory of the first embodiment.

FIG. 7 is a sectional view showing a method for manufacturing a magneticrandom access memory of the first embodiment.

FIG. 8 is a sectional view showing a method for manufacturing a magneticrandom access memory of the first embodiment.

FIG. 9 is a sectional view showing a method for manufacturing a magneticrandom access memory of the first embodiment.

FIG. 10 is a sectional view showing a method for manufacturing amagnetic random access memory of the first embodiment.

FIG. 11 is another example of the first embodiment of a magnetic randomaccess memory according to the present invention.

FIG. 12 is another example of the first embodiment of a magnetic randomaccess memory according to the present invention.

FIG. 13 is another example of the first embodiment of a magnetic randomaccess memory according to the present invention.

FIG. 14 is another example of the first embodiment of a magnetic randomaccess memory according to the present invention.

FIG. 15 is a plan view showing a second embodiment of a magnetic randomaccess memory according to the present invention.

FIG. 16 is a sectional view showing the second embodiment of a magneticrandom access memory according to the present invention and is asectional view taken along line B-B′ of FIG. 15.

FIG. 17 is a sectional view showing the second embodiment of a magneticrandom access memory according to the present invention and is asectional view taken along line C-C′ of FIG. 15.

FIG. 18 is a sectional view showing a third embodiment of an MRAM memorycell according to the present invention.

FIG. 19 is a plan view showing the third embodiment of an MRAM memorycell according to the present invention.

FIG. 20 is a diagram showing a dependency of a magnitude of a writecurrent required to flow in the bit line upon the quantity of offset p.

FIG. 21 is a diagram showing a dependency of a magnitude of a writecurrent required to flow in the bit line upon the width W of the bitline.

FIG. 22 is a plan view showing a variation example of the thirdembodiment of an MRAM memory cell according to the present invention.

FIG. 23 is a sectional view showing another example of the thirdembodiment of an MRAM memory cell according to the present invention.

FIG. 24 is a sectional view showing a fourth embodiment of an MRAMmemory cell according to the present invention.

FIG. 25 is a plan view showing the fourth embodiment of an MRAM memorycell according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

As shown in FIG. 2, in a first embodiment of MRAM according to thepresent invention, an interlayer insulator film 2 is formed on the mainsurface 1 a of a substrate 1. A word line 3 is formed on the interlayerinsulator film 2. As shown in FIG. 3, the word line 3 is extended in thex-axis direction substantially parallel with the main surface 1 a of thesubstrate 1.

As shown in FIG. 2, the interlayer insulator film 2 and the word line 3are covered with an interlayer insulator film 4. The interlayerinsulator film 4 has wiring forming faces 4 a, 4 b and amagnetoresistance element forming face 4 c. The wiring forming faces 4a, 4 b and the magnetoresistance element forming face 4 c aresubstantially parallel with the main surface 1 a of the substrate 1. Themagnetoresistance element forming face 4 c is more distant from the mainsurface 1 a of the substrate 1 than the wiring forming faces 4 a and 4b. The word line 3 is located between the magnetoresistance elementforming face 4 c and the main surface 1 a of the substrate 1.

A magnetoresistance element 5 is formed on the magnetoresistance elementforming face 4 c of the interlayer insulator film 4. Themagnetoresistance element 5 comprises a lower ferromagnetic layer 6, atunnel insulator layer 7 and an upper ferromagnetic layer 8. The lowerferromagnetic layer 6, the tunnel insulator layer 7 and the upperferromagnetic layer 8 form a magnetic tunnel junction. As shown in FIG.3, the upper ferromagnetic layer 8 is in the shape of an ellipse havingthe major axis substantially in the x-axis direction in case of beingseen from a direction perpendicular to the main surface 1 a of thesubstrate 1. The lower ferromagnetic layer 6 extends in the x-axisdirection and is connected to the word line 3 through a contact formedat a position not illustrated.

FIG. 4 shows a sectional structure of the magnetoresistance element 5.The lower ferromagnetic layer 6 of the magnetoresistance element 5comprises a first tantalum layer 6 a, an aluminum layer 6 b, a secondtantalum layer 6 c, an initial ferromagnetic layer 6 d, anantiferromagnetic layer 6 e and a fixed ferromagnetic layer 6 f whichare stacked in order. The first tantalum layer 6 a and the secondtantalum layer 6 c are formed out of tantalum. The aluminum layer 6 b isformed out of aluminum. The initial ferromagnetic layer 6 d is formedout of permalloy. The antiferromagnetic layer 6 e is formed out ofIr—Mn. The fixed ferromagnetic layer 6 f is formed out of Co—Fe. Thefixed ferromagnetic layer 6 f has a spontaneous magnetization and thedirection of the spontaneous magnetization is fixed by the interactionreceived from the antiferromagnetic layer 6 e.

The tunnel insulator layer 7 is formed on the fixed ferromagnetic layer6 f of the lower ferromagnetic layer 6. The tunnel insulator layer 7 isso thin that a tunnel current flow in the thickness direction (z-axisdirection), and the thickness of the tunnel insulator layer 7 istypically 1 to 3 nm. An alumina (Al₂O₃) film made by a plasma oxidationmethod is used as the tunnel insulator layer 7.

The upper ferromagnetic layer 8 is formed on the tunnel insulator layer7. The upper ferromagnetic layer 8 comprises a free ferromagnetic layer8 a and a tantalum layer 8 b formed on the free ferromagnetic layer 8 a.As shown in FIG. 3, the free ferromagnetic layer 8 a has a spontaneousmagnetization 8 c being freely invertible in the +x direction or the −xdirection. In the magnetic random access memory, data is stored as adirection of the spontaneous magnetization 8 c of the free ferromagneticlayer 8 a. Referring to FIG. 4, the resistance in the thicknessdirection of the tunnel insulator layer 7 varies according to adirection of the spontaneous magnetization 8 c of the free ferromagneticlayer 8 a. It is possible to discriminate data stored in the saidmagnetic random access memory by detecting a change of this resistance.In an illustrative embodiment, the free ferromagnetic layer 8 a isformed out of Ni—Fe, and the tantalum layer 8 b is formed out oftantalum.

The magnetoresistance element forming face 4 c of the interlayerinsulator film 4 and the magnetoresistance element 5 are covered with aninterlayer insulator film 9. The interlayer insulator film 9 is joinedto a side face of the magnetoresistance element 5. A cap layer 10 isformed so as to pass through the interlayer insulator film 9 and reachthe upper ferromagnetic layer 8. Wiring forming faces 4 a and 4 b of theinterlayer insulator film 4 are not covered with the interlayerinsulator film 9.

A bit line 11 is provided so as to pass above the magnetoresistanceelement 5 and extend in the y-axis direction. The y-axis direction is adirection being substantially parallel with the main surface 1 a of thesubstrate 1 and substantially perpendicular to the x-axis direction. Thebit line 11 is insulated by the interlayer insulator film 9 from thelower ferromagnetic layer 6 of the magnetoresistance element 5. The bitline 11 is electrically connected to the upper ferromagnetic layer 8 ofthe magnetoresistance element 5 through the cap layer 10.

In case that data is written, a write current is made to flow in theword line 3 and the bit line 11. The write current made to flow in theword line 3 generates a magnetic field in a direction perpendicular tothe spontaneous magnetization 8 c possessed by the free ferromagneticlayer 8 a of the magnetoresistance element 5. When a magnetic field isapplied in a direction perpendicular to the spontaneous magnetization 8c, the coercive magnetic field of the free ferromagnetic layer 8 a ismade small and the inversion of the spontaneous magnetization 8 c of thefree ferromagnetic layer 8 a is made easy. When a write current is madeto flow in the bit line 11 in this state, the write current generates amagnetic field in a direction parallel or anti-parallel with thespontaneous magnetization possessed by the free ferromagnetic layer 8 aof the magnetoresistance element 5 and inverts the spontaneousmagnetization 8 c into a desired direction.

In order to reduce a write current made to flow in the bit line 11, thebit line 11 is formed so as to have the following structure. The bitline 11 is formed so as to comprise: (1) a horizontal wiring portion 11a which is formed on the interlayer insulator film 4, extends in they-axis direction and reaches a first position 12 a; (2) a verticalwiring portion 11 b which extends along a side face of the interlayerinsulator film 9 from the first position 12 a in a direction (the z-axisdirection) substantially perpendicular to the main surface 1 a of thesubstrate 1 and reaches a second position 12 b; (3) a horizontal wiringportion 11 c which passes through a third position 12 c being above themagnetoresistance element 5 from the second position 12 b, extends inthe y-axis direction and reaches a fourth position 12 d; (4) a verticalwiring portion 11 d which extends in the z-axis direction from thefourth position 12 d and reaches a fifth position 12 e; and (5) ahorizontal wiring portion 11 e which is formed on the interlayerinsulator film 4 and extends in the y-axis direction from the fifthposition 12 e. The first position 12 a and the fifth position 12 e arecloser to the main surface 1 a of the substrate 1 than themagnetoresistance element 5 and are located so as to put themagnetoresistance element 5 between them in case of being seen fromabove the substrate 1.

When a write current is made to flow in the bit line 11 having such astructure, as shown in FIG. 3, a magnetic field 13 a is generated by thevertical wiring portion 11 b extending in the z-axis direction, amagnetic field 13 b is generated by the horizontal wiring portion 11 cextending in the y-axis direction, and a magnetic field 13 c isgenerated by the vertical wiring portion 11 d extending in the z-axisdirection. The magnetic fields 13 a to 13 c are added to one anothersince they coincide in direction with one another in themagnetoresistance element 5 and the resultant large magnetic field isapplied to the magnetoresistance element 5. In a conventional MRAM, onlya magnetic field corresponding to the magnetic field 13 b generated bythe horizontal wiring portion 11 c extending in the y-axis direction isutilized to write data. The above-mentioned structure of the bit line 11utilizes effectively a write current and makes it possible to apply alarge magnetic field to the magnetoresistance element 5. Therefore, itis possible to write data by means of a smaller write current.

Referring to FIG. 2, a fact that the vertical wiring portion 11 b andthe vertical wiring portion 11 d of the bit line 11 extend in adirection perpendicular to the main surface 1 a of the substrate 1 (thez-axis direction) is preferable in that a write current can be moreeffectively utilized. The vertical wiring portion 11 b and the verticalwiring portion 11 d can be made closer to the magnetoresistance element5 by extending the vertical wiring portion 11 b and the vertical wiringportion 11 d in the z-axis direction. Due to a fact that the verticalwiring portion 11 b and the vertical wiring portion 11 d are made closerto the magnetoresistance element 5, a larger magnetic field is appliedto the magnetoresistance element 5 and a write current can be furtherreduced.

Such a structure is effective for a micro-fabricated MRAM, and is moreeffective particularly for an MRAM micro-fabricated to such a degreethat the area of a memory cell is made small and the length of thehorizontal wiring portion 11 c (namely, the distance between the twoends of the upper face of the interlayer insulator film 9 in the y-axisdirection) is made smaller than the length of each of the verticalwiring portions 11 b and 11 d (namely, the distance between the upperface of the interlayer insulator film 9 and the wiring forming face 4 aof the interlayer insulator film 4). In an MRAM micro-fabricated to sucha degree, a part of a magnetic field to be applied to themagnetoresistance element 5, said part being provided by the verticalwiring portions 11 b and 11 d, is made large and a write current can beeffectively reduced.

A fact that the first position 12 a and the fifth position 12 e throughwhich the bit line passes are located closer to the substrate 1 than themagnetoresistance element 5 is preferable in that a write operation ofdata can be performed by means of a smaller write current. Due to a factthat the first position 12 a and the fifth position 12 e are locatedcloser to the substrate 1 than the magnetoresistance element 5, thecomponent of a write current flowing perpendicularly to the main surface1 a of the substrate 1 is increased. Due to this, a write current ismore effectively utilized and a write operation of data can be performedby means of a smaller write current.

The closer to the substrate 5 the first position 12 a and the fifthposition 12 e are, the larger the component becomes flowingperpendicularly to the main surface 1 a of the substrate 1 of a writecurrent made to flow in the bit line 11. It is preferable that the firstposition 12 a and the fifth position 12 e should be made close to thesubstrate 1 to such a degree that a half or more of the intensity of amagnetic field to be applied to the magnetoresistance element 5 isprovided by the vertical current component. Such a structure makes itpossible to more effectively utilize a write current and perform a writeoperation of data by means of a smaller write current.

FIGS. 5 to 10 show a method for manufacturing an MRAM of the firstembodiment of the present invention. This MRAM manufacturing method isstarted at a process of forming an interlayer insulator film 2 on asubstrate 1, as shown in FIG. 5. A word line 3 is formed on theinterlayer insulator film 2. Subsequently, as shown in FIG. 6, aninterlayer insulator film 4 is formed on the word line 3, and then thesurface of the interlayer insulator film 4 is made flat by CMP (chemicalmechanical polishing).

After the interlayer insulator film 4 has been made flat, as shown inFIG. 7, a magnetoresistance element 5 is formed on the interlayerinsulator film 4. The formation of the magnetoresistance element 5 isperformed by a method known by people in this technical field and is notdescribed in detail. Subsequently, as shown in FIG. 8, an interlayerinsulator film 9 is formed on the interlayer insulator film 4 and themagnetoresistance element 5 and then the surface of the interlayerinsulator film 9 is made flat by CMP.

Subsequently, as shown in FIG. 9, a contact hole penetrating theinterlayer insulator film 9 and reaching the magnetoresistance element 5is formed and thereafter a cap film 10 in the contact hole.

After the cap layer 10 is formed, as shown in FIG. 10, the interlayerfilm 9 and the interlayer film 4 are etched. The etching of theinterlayer insulator film 4 is stopped halfway in the interlayerinsulator film 4, and thereby, on the interlayer insulator film 4 thereare formed wiring forming faces 4 a, 4 b and a magnetoresistance elementforming face 4 c being more distant from the main surface 1 a of thesubstrate 1 than the wiring forming faces 4 a, 4 b. Due to a fact thatthe interlayer film 9 and the interlayer film 4 are etched, a part of abit line 11 extending perpendicularly the main surface 1 a of thesubstrate 1 is made longer and a structure intensifying a magnetic fieldto be applied to the magnetoresistance element 5 is realized.Subsequently, the bit line 11 is formed along the wiring forming faces 4a, 4 b and the side and upper faces of the interlayer insulator film 9,and the MRAM shown in FIG. 2 is formed.

As described above, in the MRAM of the first embodiment, due to theabove-mentioned structure of the bit line 11, a horizontal currentcomponent flowing in a horizontal direction relative to the main surface1 a and a vertical current component flowing in a directionperpendicular to the main surface 1 a of the substrate 1 are generatedin the vicinity of the magnetoresistance element 5. A magnetic fieldgenerated by the horizontal current component and a magnetic fieldgenerated by the vertical current component are added to each othersince they coincide in direction with each other in themagnetoresistance element 5, and the resultant large magnetic field isapplied to the magnetoresistance element 5. Thus, a write operation ofdata can be performed by means of a smaller write current.

In the first embodiment, as shown in FIG. 11, the etching of theinterlayer insulator film 9 and the interlayer insulator film 4 can beperformed so that the side faces formed by the etching are inclined.Such a structure makes obtuse the angle of bending the bit line 11,effectively prevents the breaking of wire in the bit line 11, andfurther improves the tolerance to electromigration of the bit line 11.

In the first embodiment, as shown in FIG. 12, it is also possible that abit line 11 extended in the y-axis direction (namely, a directionperpendicular to the spontaneous magnetization of the upperferromagnetic layer 8) is formed below the magnetoresistance element 5and a word line 3 extended in the x-axis direction is formed above themagnetoresistance element 5, and vertical wiring portions 3 a, 3 bextended in a direction perpendicular to the main surface of thesubstrate 1 are provided in the word line 3.

As shown in FIG. 2, however, a structure in which vertical wiringportions 11 b and 11 d extended in a direction perpendicular to the mainsurface of the substrate 1 provided in the bit line extended in they-axis direction is more preferable. Since the upper ferromagnetic layer8 is formed so as to have the major axis 8 c in the x-axis direction(namely, the direction of spontaneous magnetization of the upperferromagnetic layer 8), in the structure of FIG. 12 the distance betweenthe center of the upper ferromagnetic layer 8 and the vertical wiringportion 3 a or 3 b of the word line 3 extended in the x-axis directionis made large. Therefore, even if the vertical wiring portions 3 a and 3b are provided in the word line 3, magnetic fields generated by thevertical wiring portions 3 a and 3 b are difficult to reach the centerof the upper ferromagnetic layer 8. This means that the efficiency ofintensifying the magnetic field by the vertical wiring portions 3 a and3 b is low. On the other hand, in the structure of FIG. 2, the distancebetween the vertical wiring portion 11 b or 11 d of the bit line 11extended in a vertical direction and the center of the upperferromagnetic layer 8 is made short and the effect of intensifying themagnetic field appears on the whole upper ferromagnetic layer 8. Asshown in FIG. 2, therefore, a structure in which the vertical wiringportion 11 b and 11 d are provided in the bit line 11 extended in adirection perpendicular to the spontaneous magnetization of the upperferromagnetic layer 8 is preferable.

Referring to FIG. 13, the upper ferromagnetic layer 8 is symmetric withrespect to a plane of symmetry 8 f being substantially parallel with they-axis direction and substantially perpendicular to the main surface 1 aof the substrate 1, but the center line 11 f of the bit line 11 ispreferably arranged so as to be offset in the x-axis direction relativeto the plane of symmetry 8 f of the upper ferromagnetic layer 8. Sucharrangement of the bit line 11 makes it possible to further reduce awrite current. The effect of reducing a write current obtained due to afact that the center line 11 f of the bit line 11 is offset in thex-axis direction relative to the minor axis 8 d of the upperferromagnetic layer 8 is estimated to be caused by the followingmechanism.

A domain contained in the free ferromagnetic layer 8 a of the upperferromagnetic layer 8 receives an exchange interaction to make theadjacent domains uniform in direction of magnetization. Due to thisexchange interaction, the inversion of spontaneous magnetization 8 c ofthe free ferromagnetic layer 8 a shows a behavior that the inversion ofspontaneous magnetization is started at domains in the outer edge of thefree ferromagnetic layer 8 a and then propagates to domains in themiddle of it. Domains in the middle of the free ferromagnetic layer 8 aare hindered from being inverted by receiving the exchange interactionfrom all domains existing around them. On the other hand, since domainsin the outer edge of the free ferromagnetic layer 8 a have regions beingnot adjacent to other domains, they receive a small exchange interactionfrom the surrounding domains and are inverted by a comparatively smallmagnetic field. When the domains in the outer edge are inverted, theinversion of domains adjacent to those domains is also facilitated andthey are inverted. In such a way, the inversion of domains is started atthe outer edge and propagates to the middle region.

By making the center line 11 f of the bit line 11 offset from the planeof symmetry 8 f of the upper ferromagnetic layer 8, the position where amagnetic field generated by the bit line 11 becomes the maximum is madecloser to the outer edge of the free ferromagnetic layer 8 a and it ispossible to invert domains existing in the outer edge of the freeferromagnetic layer 8 a by means of a smaller write current. If domainsin the outer edge of the free ferromagnetic layer 8 a are inverted, theinversion of domains propagates from the outer edge to the center andthe spontaneous magnetization 8 c of the free ferromagnetic layer 8 a iscompletely inverted. Hence, by making the center line 11 f of the bitline 11 offset from the plane of symmetry 8 f of the upper ferromagneticlayer 8 and making the center line 1 if of the bit line 11 closer to theouter edge of the free ferromagnetic layer 8 a, it is possible to invertthe spontaneous magnetization 8 c of the free ferromagnetic layer 8 a bymeans of a smaller write current.

It is preferable that the bit line 11 should have a portion whichprotrudes in the x-axis direction from an end of the major axis 8 e ofthe upper ferromagnetic layer 8 (namely, a segment 8 e tying both endsin the x-direction of the upper ferromagnetic layer 8 being in the shapeof an ellipse) and does not overlap the upper ferromagnetic layer 8 incase of being seen from a direction perpendicular to the main surface 1a of the substrate 1. Such arrangement of the bit line 11 is preferablein the regard of making it possible to invert the spontaneousmagnetization 8 c of the free ferromagnetic layer 8 a by means of asmaller write current. The arrangement of the bit line 11 protruding inthe x-axis direction from the upper ferromagnetic layer 8 makes theposition where a magnetic field generated by the bit line 11 becomes themaximum closer to the outer edge of the free ferromagnetic layer 8 a.Therefore, a write current necessary for inverting domains in the outeredge of the free ferromagnetic layer 8 a is made smaller and as aresult, the magnitude of a write current necessary for inverting thespontaneous magnetization 8 c of the free ferromagnetic layer 8 a isalso made smaller.

Providing a width W of the bit line 11 in the x-axis direction which isnarrower than the length L of the upper ferromagnetic layer 8 in thex-axis direction (namely, length L of the major axis 8 e) is preferablein the regard of making it possible to invert the spontaneousmagnetization 8 c of the free ferromagnetic layer 8 a by means of asmaller write current. This effect is brought by a fact that when thewidth W of the bit line 11 is made narrower, a magnetic field generatedby the bit line 11 concentrates at domains in the outer edge of the freeferromagnetic layer 8 a. Due to a fact that a magnetic field generatedby the bit line 11 concentrates at domains in the

The word line 24 is formed so as to extend in the x-axis direction. Theword line 24 is formed along the upper and side faces of the hollowforming insulator film 23 and the upper face of the interlayer insulatorfilm 22. That is to say, the word line 24 is formed so as to comprise:

(1) a horizontal wiring portion 24 a which is formed on the hollowforming insulator film 23 and extends in the x-axis direction along theupper face of the interlayer insulator film 23; (2) a vertical wiringportion 24 b which is connected with the horizontal wiring portion 24 a,extends from the position of connection with the wiring portion 24 aalong a side face of the hollow forming insulator film 23 in a directionsubstantially perpendicular to the main surface 1 a of the substrate 1(the z-axis direction), and reaches the interlayer insulator film 22;(3) a horizontal wiring portion 24 c which is connected with thevertical wiring portion 24 b and extends along the upper face of theinterlayer insulator film 22 in the x-axis direction; (4) a verticalwiring portion 24 d which is connected with the horizontal wiringportion 24 c and extends along a side face of the hollow forminginsulator film 23 in a direction substantially perpendicular to the mainsurface 1 a of the substrate 1 (the z-axis direction); and (5) ahorizontal wiring portion 24 e which is connected with the verticalwiring portion 24 d and extends along the upper face of the hollowforming insulator film 23 in the x-axis direction.

Such a structure of the word line 24 makes it possible to moreeffectively utilize a write current, apply a large magnetic field to themagnetoresistance element 27 and write data by means of a smaller writecurrent. When a write current is made to flow in the word line 24, thevertical wiring portion 24 b, the horizontal wiring portion 24 c and thevertical wiring portion 24 d each generate a magnetic field. Therespective magnetic fields generated by the vertical wiring portion 24b, the horizontal wiring portion 24 c and the vertical wiring portion 24d coincide substantially in direction with one another in themagnetoresistance element 27. Therefore, the magnetic fields generatedby the vertical wiring portion 24 b, the horizontal wiring portion 24 cand the vertical wiring portion 24 d are added to one another and theresultant large magnetic field is applied to the magnetoresistanceelement 27. According to this, a write current is more effectivelyutilized and a large magnetic field is applied to the magnetoresistanceelement 27. Therefore, a write operation of data can be performed bymeans of a smaller write current.

An interlayer insulator film 25 is formed so as to cover the horizontalwiring portion 24 c of the word line 24. With reference to FIG. 17showing a sectional structure taken along by line C-C′ (a sectionpassing through the magnetoresistance element 27 and being parallel withthe y-z plane), the interlayer insulator film 25 has wiring formingfaces 25 a, 25 b and a magnetoresistance element forming face 25 c. Thewiring forming faces 25 a, 25 b and the magnetoresistance elementforming face 25 c are substantially parallel with the main surface 21 aof the substrate 21. The magnetoresistance element forming face 25 c ismore distant from the main surface 21 a of the substrate 21 than thewiring forming faces 25 a, 25 b. The horizontal wiring portion 24 c islocated between the magnetoresistance element forming face 24 c and themain surface 21 a of the substrate 21.

An electrically conductive contact 26 reaching the word line 24 from themagnetoresistance element forming face 25 c is formed in the interlayerinsulator film 25. The magnetoresistance element 27 is formed on themagnetoresistance element forming face 25 c. The magnetoresistanceelement 27 is electrically connected with the word line 24 through thecontact 26. The magnetoresistance element 27 comprises a lowerferromagnetic layer 28, a tunnel insulator layer 29 and an upperferromagnetic layer 30. Sectional structures of the lower ferromagneticlayer 28, the tunnel insulator layer 29 and the upper ferromagneticlayer 30 are the same as the sectional structures of the lowerferromagnetic layer 6, the tunnel insulator layer 7 and the upperferromagnetic layer 8 of the magnetoresistance element 5 of the firstembodiment. The lower ferromagnetic layer 28 comprises a fixedferromagnetic layer having a spontaneous magnetization 8 c fixed indirection and the upper ferromagnetic layer 28 comprises a freeferromagnetic layer having an invertible spontaneous magnetization 8 c.As shown in FIG. 15, the magnetoresistance element 27 is in the shape ofan ellipse being long substantially in the x-direction in case of beingseen from above the substrate 21.

As shown in FIG. 17, the magnetoresistance element forming face 25 c ofthe interlayer insulator film 25 and the magnetoresistance element 27are covered with an interlayer insulator film 31. The interlayerinsulator film 31 is joined to the side faces of the magnetoresistanceelement 27. A cap layer 32 is formed so as to pass through theinterlayer insulator film 31 and reach the upper ferromagnetic layer 30.Wiring forming faces 25 a and 25 b are not covered with the interlayerinsulator film 31.

A bit line 33 is provided so as to pass above the magnetoresistanceelement 27 and extend in the y-axis direction. The bit line 33 isinsulated from the lower ferromagnetic layer 28 of the magentoresistanceelement 27 by the interlayer insulator film 31. The bit line 33 iselectrically connected with the upper ferromagnetic layer 30 of themagnetoresistance element 27 through the cap layer 32.

The structure of the bit line 33 is similar to the structure of the bitline 11 of the first embodiment. The bit line 33 is formed along thewiring forming faces 25 a, 25 b of the interlayer insulator film 25 andthe side and upper faces of the interlayer insulator film 31. That is tosay, the bit line 33 is formed so as to comprise: (1) a horizontalwiring portion 33 a which is formed on the wiring forming face 25 a ofthe interlayer insulator film 25 and extends in the y-axis directionalong the wiring forming face 25 a; (2) a vertical wiring portion 33 bwhich is connected with the horizontal wiring portion 33 a and extendsfrom the position of connection with the horizontal wiring portion 33 ain a direction substantially perpendicular to the main surface 1 a ofthe substrate 1 (the z-axis direction) along a side wall of theinterlayer insulator film 31; (3) a horizontal wiring portion 33 c whichis connected with the vertical wiring portion 33 b and extends in they-axis direction along the upper face of the interlayer insulator film31; (4) a vertical wiring portion 33 d which is connected with thehorizontal wiring portion 33 c and extends in a direction substantiallyperpendicular to the main surface 1 a of the substrate 1 (the z-axisdirection) along a side wall of the interlayer insulator film 31; and(5) a horizontal wiring portion 33 e which is connected with thevertical wiring portion 33 d and extends in the y-axis direction alongthe upper face of the wiring forming face 25 b of the interlayerinsulator film 25.

Similar to the bit line 11 of the first embodiment, such a structure ofthe bit line 33 makes it possible to more effectively utilize a writecurrent and apply a large magnetic field to the magnetoresistanceelement 27. Due to this, it is possible to perform a write operation ofdata by means of a small write current.

As described above, in an MRAM of the second embodiment, due to thestructure of the bit line 33, a write current made to flow in the bitline 33 has a horizontal current component flowing in a horizontaldirection relative to the main surface 21 a of the substrate 21 and avertical current component flowing in a direction perpendicular to themain surface 21 a. Since a magnetic field generated by the horizontalcurrent component and a magnetic field generated by the vertical currentcomponent coincide in direction with each other in the magnetoresistanceelement 27, they are added to each other and the resultant largemagnetic field is applied to the magnetoresistance element 27. Due tothis, a write current required to flow in the bit line 33 for writingdata is made small.

Further, due to the structure of the word line 24, a write current madeto flow in the word line 24 has a horizontal current component flowingin a horizontal direction relative to the main surface 21 a of thesubstrate 21 and a vertical current component flowing in a directionperpendicular to the main surface 21 a of the substrate 21 in thevicinity of the magnetoresistance element 27. Since a magnetic fieldgenerated by the horizontal current component and a magnetic fieldgenerated by the vertical current component coincide in direction witheach other in the magnetoresistance element 27, they are added to eachother and the resultant large magnetic field is applied to themagnetoresistance element 27. Due to this, a write current required toflow in the word line 24 for writing data is made small.

Third Embodiment

FIG. 18 shows a third embodiment of an MRAM according to the presentinvention. In the third embodiment, an interlayer insulator film 2 isformed on the main surface 1 a side of a substrate 1. A word line 3 isformed on the interlayer insulator film 2. The word line 3 is providedso as to extend in the x-axis direction substantially parallel with themain surface 1 a of the substrate 1. The word line 3 is covered with aninterlayer insulator film 4. In the interlayer insulator film 4, thereis formed a conductive contact 5 penetrating the interlayer insulatorfilm 4 to reach the word line 3. A magnetoresistance element 6 is formedon the interlayer insulator film 4.

The magnetoresistance element 6 comprises a fixed ferromagnetic layer 7,a tunnel insulating layer 8 and a free ferromagnetic layer 9. The fixedferromagnetic layer 7 is electrically connected through the contact 5 tothe word line 3. The fixed ferromagnetic layer 7 has a spontaneousmagnetization fixed in the x-axis direction and the free ferromagneticlayer 9 has a spontaneous magnetization being freely invertible inparallel with the x-axis direction. The spontaneous magnetization of thefree ferromagnetic layer 9 is allowed to take a “parallel” state whereit is directed in the same direction as that of the spontaneousmagnetization of the fixed ferromagnetic layer 7 and an “anti-parallel”state where it is directed in the opposite direction. A memory cell ofthe first embodiment stores data as the direction of spontaneousmagnetization of the free ferromagnetic layer 9. The tunnel insulatinglayer 8 is made of an insulating material such as alumina (Al₂O₃) andthe thickness of the tunnel insulating layer 8 is so thin that a tunnelcurrent flows in the direction of thickness. The resistance in thedirection of thickness of the tunnel insulating layer 8 (namely, theresistance of the magnetoresistance element 6) changes according to thedirection of spontaneous magnetization of the free ferromagnetic layer9, and the change in resistance of the magnetoresistance element 6enables data stored in the magnetoresistance element 6 to bediscriminated.

As shown in FIG. 19, the free ferromagnetic layer 9 of themagnetoresistance element 6 is substantially elliptic in shape in caseof being seen from a direction perpendicular to the main surface 1 a ofthe substrate 1. The free ferromagnetic layer 9 having an elliptic shapehas the major axis 9 a in the x-axis direction and the minor axis 9 b inthe y-axis direction being substantially parallel with the primarysurface 1 a of the substrate 1 and substantially perpendicular to thex-axis direction. The spontaneous magnetization of the freeferromagnetic layer 9 is directed in parallel with the major axis 9 a.The free ferromagnetic layer 9 having such a shape is substantiallysymmetric with regard to a plane of symmetry 9 c being perpendicular tothe main surface 1 a of the substrate 1 and having the minor axis 9 b onit.

As shown in FIG. 18, the magnetoresistance element 6 is covered with aninterlayer insulator film 10. In the interlayer insulator film 10, thereis formed a conductive contact 11 reaching the free ferromagnetic layer9. A bit line 12 is formed on the interlayer insulator film 10. The bitline 12 extends in the y-axis direction.

As shown in FIG. 19, the center line 12 a of the bit line 12 is arrangedso as to be offset in the x-axis direction from the plane of symmetry 9c of the free ferromagnetic layer 9. Further, the bit line 12 has a partwhich protrudes in the x-axis direction from one end of the major axis 9a of the free ferromagnetic layer 9 in case of being seen from adirection perpendicular to the main surface 1 a of the substrate 1 anddoes not overlap the free ferromagnetic layer 9. Furthermore, the widthW of the bit line 12 in the x-axis direction is narrower than the lengthof the major axis 9 a of the free ferromagnetic layer 9 (namely, thelength of the free ferromagnetic layer 9 in the x-axis direction).

In case of writing data into the memory cell, a write current is made toflow in the word line in the x-axis direction and a write current ismade to flow in the bit line in the y-axis direction according to thedata to be written. The write current flowing in the word line 3 appliesa magnetic field to the free ferromagnetic layer 9 in the y-axisdirection. A magnetic coercive field in the free ferromagnetic layer 9is made small by applying a magnetic field in the y-axis direction andthe inversion of the free ferromagnetic layer 9 is facilitated. When awrite current is made to flow in the bit line 12 in this state, thiswrite current generates a magnetic field in the x-axis direction andinverts the spontaneous magnetization of the free ferromagnetic layer 9in a direction corresponding to the data to be written.

The above-described arrangement of the bit line 12 makes it possible towrite data by means of a smaller write current. FIGS. 20 and 21 show theeffect of reducing a write current. FIG. 20 shows dependency of themagnitude of a write current required to flow in the bit line 12 forwriting data upon the quantity of offset p between the center line 12 aof the bit line 12 and the plane of symmetry 9 c of the freeferromagnetic layer 9. The quantity of offset p is defined as: p=d/L,where d is the distance between the center line 12 a of the bit line 12and the plane of symmetry 9 c and L is a length of the freeferromagnetic layer 9 in the x-axis direction. In case of “p=0”, thecenter line 12 a of the bit line 12 is on the plane of symmetry 9 c ofthe free ferromagnetic layer 9, and the larger the quantity of offset pis, the more the center line 12 a of the bit line 12 is offset from theplane of symmetry 9 c of the free ferromagnetic layer 9. A diagram ofFIG. 20 has been obtained by the simulation performed under thecondition that the width W of the bit line 12 is half the length L ofthe free ferromagnetic layer 9.

As shown in FIG. 20, it is possible to write data by means of a smallerwrite current when the center line 12 a of the bit line 12 is offsetfrom the plane of symmetry 9 c of the free ferromagnetic layer 9 than awrite current when the center line 12 a of the bit line 12 is aligned inposition with the plane of symmetry 9 c of the free ferromagnetic layer9 (namely, p is zero). When the quantity of offset p is in the vicinityof 0.375, a necessary write current is made minimum in magnitude. Thereason for this result is as follows.

A domain contained in the free ferromagnetic layer 9 is subject to aninteraction attempting to arrange properly the direction ofmagnetization of an adjacent domain. According to this interaction, theinversion of spontaneous magnetization of the free ferromagnetic layer 9is started at a domain at the outer edge of the free ferromagnetic layer9 and thereafter exhibits a behavior of propagating to domains in themiddle. A domain in the middle of the free ferromagnetic layer 9receives interaction from all domains existing around it and isprevented from its inversion. On the other hand, since a domain at theouter edge of the free ferromagnetic layer 9 has an area having noadjacent domain, it receives a less interaction from domains surroundingit and is inverted by a comparatively small magnetic field. When adomain at the outer edge is inverted, a domain adjacent to this domainis also easily inverted. In this way, the inversion of domain is startedat the outer edge and propagated to the center.

By making the center line 12 a of the bit line 12 offset from the planeof symmetry 9 c of the free ferromagnetic layer 9, the position where amagnetic field generated by the bit line 12 is made maximum moves closeto the outer edge of the free ferromagnetic layer 9 and it becomespossible to generate the inversion of domain at the outer edge of thefree ferromagnetic layer 9 by means of a smaller write current. Byinverting a domain existing at the outer edge of the free ferromagneticlayer 9, the inversion of domain propagates from the outer edge to thecenter and the spontaneous magnetization of the free ferromagnetic layer9 is completely inverted. Therefore, by making the center line 12 a ofthe bit line 12 offset from the plane of symmetry 9 c of the freeferromagnetic layer 9 to move the center line 12 a of the bit line 12close to the outer edge of the free ferromagnetic layer 9, it ispossible to invert the spontaneous magnetization of the freeferromagnetic layer 9 by means of a smaller write current.

However, when the quantity of offset p is made too large, a magneticfield interlinked with the free ferromagnetic layer 9 is made small andinversely, a write current is increased. Therefore, it is preferablethat the quantity of offset p is not less than 0.1 and not more than0.5.

The arrangement of the bit line 12 having a part which protrudes in thex-axis direction from one end of the major axis 9 a of the freeferromagnetic layer 9 in case of being seen from a directionperpendicular to the main surface 1 a of the substrate 1 is preferablein that this arrangement makes it possible to invert the spontaneousmagnetization of the free ferromagnetic layer 9 by means of a furthersmaller write current. The arrangement of the bit line 12 protruding inthe x-axis direction from the free ferromagnetic layer 9 moves theposition where a magnetic field generated by the bit line 12 is mademaximum closer to the outer edge of the free ferromagnetic layer 9.Therefore, a write current necessary for the inversion of domain at theouter edge of the free ferromagnetic layer 9 is made small and as aresult, a write current necessary for the inversion of spontaneousmagnetization of the free ferromagnetic layer 9 is also made small inmagnitude.

Above mentioned U.S. Pat. No. 6,236,590 and Japanese publishedapplication 2002-118239A have disclosed that a misalignment between awiring in which a write current of data is made to flow and a datastorage layer (corresponding to the free ferromagnetic layer 9 of thisembodiment) makes a magnetic field leak out and increases a writecurrent. However, this study is wrong since it lacks detailedexamination of a mechanism of inverting the spontaneous magnetization ofa ferromagnetic material.

A fact that the width W of the bit line in the x-axis direction isnarrower than the length L of the free ferromagnetic layer 9 in thex-axis direction is preferable in that the spontaneous magnetization ofthe free ferromagnetic layer 9 can be inverted by a smaller writecurrent. FIG. 21 shows the dependency of the magnitude of a writecurrent required to flow in the bit line 12 for writing data upon W/L.As shown by curve A in FIG. 21, in case that the center line 12 a of thebit line 12 is not offset from the minor axis 9 b of the freeferromagnetic layer 9, like the referenced, the effect of reducing awrite current due to making less the width W of the bit line 12 is notlarge. On the other hand, in the invention, as shown by curve B in FIG.21, in case that the center line 12 a of the bit line 12 is offset fromthe plane of symmetry 9 c of the free ferromagnetic layer 9, making lessthe width W of the bit line 12 can remarkably reduce a write current.

It is conceivable that this effect is caused by a fact that when thewidth W of the bit line 12 is made narrow a magnetic field generated bythe bit line 12 concentrates on domains at the outer edge of the freeferromagnetic layer 9. A write current necessary for inverting domainsat the outer edge of the free ferromagnetic layer 9 is reduced by a factthat a magnetic field generated by the bit line 12 concentrates on thedomains at the outer edge of the free ferromagnetic layer 9. As aresult, it is conceivable that the magnitude of a write currentnecessary for inverting the spontaneous magnetization of the freeferromagnetic layer 9 is also reduced.

However, a too small width W of the bit line 12 is not preferable forthe reason that it deteriorates the tolerance to electromigration.Therefore, it is preferable that the width W of the bit line 12 is notless then 0.3 times and not more than 0.7 times the length L of the freeferromagnetic layer 9.

As described above, in an MRAM memory cell of this embodiment, thecenter line 12 a of the bit line 12 is arranged so as to be offset inthe x-axis direction from the plane of symmetry 9 c of the freeferromagnetic layer 9. Further, the bit line 12 has a part whichprotrudes in the x-axis direction from one end of the major axis 9 a ofthe free ferromagnetic layer 9 and does not overlap the freeferromagnetic layer 9. Furthermore, the width W of the bit line 12 inthe x-axis direction is narrower than the length of the major axis 9 aof the free ferromagnetic layer 9 (namely, the length of the freeferromagnetic layer 9 in the x-axis direction). Such an arrangement ofthe bit line 12 reduces the magnitude of a write current required toflow in the bit line 12 at the time of writing data.

In this embodiment, by adopting a similar technique to that of the bitline 12, it is possible to reduce a write current in the word line 3. Asshown in FIG. 22, although the free ferromagnetic layer 9 issubstantially symmetrical with regard to a plane of symmetry 9 d whichis perpendicular to the main surface 1 a of a substrate 1 and has themajor axis 9 a on it, a write current in the word line 3 can be reducedby arranging the center line 3 a of the word line 3 so as to be offsetin the y-axis direction from the plane of symmetry 9 d of the freeferromagnetic layer 9. Such an arrangement of the word line 3 reducesthe magnitude of a write current required to flow in the word line 3 atthe time of writing data due to a similar mechanism to that of reductionof a write current by means of the bit line 12 as described above. Atthis time, the word line 3 having a part which protrudes in the y-axisdirection from one end of the minor axis 9 b of the free ferromagneticlayer 9 and does not overlap the free ferromagnetic layer 9 ispreferable since it makes smaller the magnitude of a write currentrequired to flow in the word line 3 for writing data. Further, a factthat the width W′ of the word line 3 in the y-axis direction is lessthan the length L′ of the minor axis 9 b of the free ferromagnetic layer9 (namely, the width of the free ferromagnetic layer 9 in the y-axisdirection) is preferable in that it makes smaller the magnitude of awrite current required to flow in the word line 3 for writing data.

Furthermore, as shown in FIG. 23, it is preferable that a magnetic layer13 having a high magnetic permeability is formed on the top face andside faces of the bit line 12. The magnetic layer 13 is typically formedout of permalloy. Such a structure concentrates a magnetic fieldgenerated by the bit line 12 on the free ferromagnetic layer 9 and makessmaller the magnitude of a write current required to flow in the wordline 3 at the time of writing data.

Fourth Embodiment

FIG. 24 shows a fourth embodiment of an MRAM memory cell according tothe present invention. In the fourth embodiment, separately from a readbit line 15 to be used in reading data, a write bit line 16 to be usedin writing data is provided in a memory cell. Both the read bit line 15and the write bit line 16 are formed on an interlayer insulator film 10.The read bit line 15 is electrically connected to a free ferromagneticlayer 9 of a magnetoresistance element 6 through a contact 14penetrating the interlayer insulator film 10 to reach the freeferromagnetic layer 9. The write bit line 16 is electrically insulatedfrom the magnetoresistance element 6.

Further, in the fourth embodiment, a contact 5 is removed, and a wordline 3 and a fixed ferromagnetic layer 7 are electrically insulated fromeach other. The fixed ferromagnetic layer 7 extends in the x-directionand is also used as a read word line. The word line 3 is usedexclusively in writing data.

In case that data is read from the memory cell, a specific voltage isapplied between the fixed ferromagnetic layer 7 to be used as a readword line and the read bit line 15. Since the resistance value of themagnetoresistance element 6 varies according to the direction ofspontaneous magnetization of the free ferromagnetic layer 9, namely,data stored in the said memory cell, an electric current flowing in themagnetoresistance element 6 varies according to the data stored in thesaid memory cell. The data stored in the said memory cell isdiscriminated on the basis of the current flowing in themagnetoresistance element 6.

In case that data is written into the memory cell, a write current ismade to flow in the word line 3 in the x-axis direction and a writecurrent is made to flow in the write bit line 16 in the y-axis directionaccording to the data to be written. The write current flowing in theword line 3 applies a magnetic field to the free ferromagnetic layer 9in the y-axis direction. The magnetic coercive field in the freeferromagnetic layer 9 is made small by applying a magnetic field in they-axis direction and the inversion of the free ferromagnetic layer 9 isfacilitated. A write current made to flow in the write bit line 16generates a magnetic field in the x-axis direction and inverts thespontaneous magnetization of the free ferromagnetic layer 9 according tothe data to be written.

A fact that the write bit line 16 is electrically insulated from theread bit line 15 and the word line 3 is electrically insulated from thefixed ferromagnetic layer 7 functioning as a read word line ispreferable in that it makes it possible to read and write data at thesame time. Further, such a structure which is preferable in that thisstructure as well makes it possible to separately provide a selector(not illustrated) for selecting a read bit line 15 and a selector forselecting a write bit line 16, and separately provide a selector forselecting a write word line and a selector for selecting a read wordline and thereby makes it possible to simplify circuits contained inthese selectors.

In a similar manner to the bit line 12 of the third embodiment, thecenter line 16 a of the write bit line 16 is arranged so as to be offsetin the x-axis direction from the plane of symmetry 9 c of the freeferromagnetic layer 9. The write bit line 16 has a part which protrudesin the x-axis direction from one end of the major axis 9 a of the freeferromagnetic layer 9 in case of being seen from a directionperpendicular to the primary surface 1 a of the substrate 1 and does notoverlap the free ferromagnetic layer 9. Further, the width W of thewrite bit line 16 in the x-axis direction is narrower than the length ofthe major axis 9 a of the free ferromagnetic layer 9 (namely, the lengthof the ferromagnetic layer 9 similarly to the bit line 12 of the firstembodiment in the x-axis direction). Such an arrangement of the writebit line 16 makes it possible to write data by means of a smaller writecurrent similarly to the bit line 11 of the first embodiment.

Providing the center line 16 a of the write bit line 16 so as to beoffset in the x-axis direction from the plane of symmetry 9 c of thefree ferromagnetic layer 9 is preferable in that it makes it possible toform both of the read bit line 15 and the write bit line 16 on theinterlayer insulator film 10, namely, form the read bit line 15 and thewrite bit line 16 by means of a single wiring layer. According toarranging the center line 16 a of the write bit line 16 so as to beoffset in the x-axis direction from the plane of symmetry 9 c of thefree ferromagnetic layer 9, a space in which the write bit line 15 is tobe arranged above the magnetoresistance element is generated on theinterlayer insulator film 10. Arranging the write bit line 15 in thisspace makes it possible to form the read bit line 15 and the write bitline 16 by means of a single wiring layer. Also, by forming the read bitline 15 and the write bit line 16 of a single wiring layer, themanufacturing process can be simplified.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention. Moreover,various modifications to these embodiments will be readily apparent tothose skilled in the art, and the generic principles and specificexamples defined herein may be applied to other embodiments without theuse of inventive faculty. Therefore, the present invention is notintended to be limited to the embodiments described herein but is to beaccorded the widest scope as defined by the limitations of the claimsand equivalents.

1-13. (canceled)
 14. A magnetic random access memory comprising: asubstrate; a magnetoresistance element being formed above saidsubstrate, having an invertible spontaneous magnetization and beingdifferent in resistance according to the direction of said spontaneousmagnetization; and a wiring for making an electric current flow togenerate a magnetic field to be applied to said magnetoresistanceelement, wherein said electric current has a parallel current componentflowing in a direction parallel with a main surface of said substrateand a perpendicular current component flowing in a directionperpendicular to the main surface of said substrate, and wherein a halfor more of the intensity of a magnetic field to be applied to saidmagnetoresistance element is provided by said perpendicular currentcomponent.
 15. A magnetic random access memory manufacturing methodcomprising: forming a first interlayer insulator film covering asubstrate, forming a magnetoresistance element which comprises aferromagnetic layer having an invertible spontaneous magnetization andvaries in resistance according to the direction of said spontaneousmagnetization on said first interlayer insulator film, forming a secondinterlayer insulator film outside of said magnetoresistance element,etching a portion not overlapping said magnetoresistance element of saidsecond interlayer insulator film, and forming a wiring for applying amagnetic field to said magnetoresistance element along the upper andside faces of said second interlayer insulator film.
 16. The magneticrandom access memory manufacturing method according to claim 15, whereina part of said first interlayer insulator film is exposed by said stepof etching a portion of said second interlayer insulator film, whereinsaid wiring is formed on said exposed part of said first interlayerinsulator film.
 17. The magnetic random access memory manufacturingmethod according to claim 16, further comprising etching said exposedpart of said first interlayer insulator film.
 18. A magnetic randomaccess memory comprising; a substrate; a ferromagnetic layer having aninvertible spontaneous magnetization, and formed above the main surfaceside of said substrate; a first wiring which extends in a firstdirection substantially parallel with said substrate and for flowing anelectric current to invert said spontaneous magnetization, wherein saidferromagnetic layer is substantially symmetric with respect to a planeof symmetry being substantially perpendicular to said substrate, whereina centerline of said first wiring is shifted to said plane of symmetry.19. The magnetic random access memory according to claim 18, whereinsaid first wiring has a portion which protrudes from an end of saidferromagnetic layer in a second direction perpendicular to said plane ofsymmetry toward said second direction and which does not overlap saidferromagnetic layer.
 20. The magnetic random access memory according toclaim 19, wherein a quantity of offset p is defined by “p=d/L”, whereinthe distance between said centerline of said first wiring and said planeof symmetry is d, the length of said free ferromagnetic layer in asecond direction perpendicular to said plane of symmetry is L, and p isnot less than 0.1 and not more than 0.5.
 21. The magnetic random accessmemory according to claim 19, wherein a width of said first wiring in asecond direction perpendicular to said plane of symmetry is narrowerthan the length of said free ferromagnetic layer in said seconddirection.
 22. The magnetic random access memory according to claim 21,wherein said width of said first wiring is not less than 0.3 times andnot more than 0.7 times said length of said free ferromagnetic layer.23. The magnetic random access memory according to claim 18, wherein thedirection of said spontaneous magnetization is parallel with said seconddirection.
 24. The magnetic random access memory according to claim 18,further comprising a magnetic layer joined to said first wiring and madeof a magnetic material.
 25. The magnetic random access memory accordingto claim 18, further comprising a second wiring which extends in saidsecond direction and in which a second write current for flowing anelectric current to invert said spontaneous magnetization wherein saidferromagnetic layer is substantially parallel with said second directionand substantially symmetrical with respect to a plane of symmetry beingsubstantially parallel with said second direction and substantiallyperpendicular to said substrate, and wherein a centerline of said secondwiring is shifted to said plane of symmetry.
 26. A magnetic randomaccess memory comprising: a substrate; a ferromagnetic layer having aninvertible spontaneous magnetization, and formed above the main surfaceside of said substrate; a first wiring which extends in a firstdirection and is electrically connected to said magnetoresistanceelement; and a second wiring which extends in said first direction andis electrically insulated from said magnetoresistance element andcurrent for flowing inverting electric current to invert saidspontaneous magnetization, wherein said ferromagnetic layer issubstantially symmetric with respect to a plane of symmetry beingsubstantially perpendicular to said substrate, and wherein a centerlineof said first wiring is shifted to said plane of symmetry.
 27. Themagnetic random access memory according to claim 26, further comprisingan interlayer insulator film outside of said magnetoresistance element,wherein said first wiring and said second wiring are formed on saidinterlayer insulator film.
 28. A magnetic random access memorycomprising: a substrate; a ferromagnetic layer having an invertiblespontaneous magnetization, and formed above the main surface side ofsaid substrate; a first wiring which extends in a first directionsubstantially parallel with said substrate and for flowing an electriccurrent to invert said spontaneous magnetization, wherein saidferromagnetic layer is substantially symmetric with respect to a planeof symmetry being substantially perpendicular to said substrate, andwherein a position where a magnetic field generated by said electriccurrent is made strongest is shifted to said plane of symmetry.
 29. Aspontaneous magnetization inversion promoting method comprising:providing a free ferromagnetic layer having an invertible spontaneousmagnetization so as to be substantially symmetric with respect to aplane of symmetry substantially perpendicular to a primary surface of asubstrate; forming a wiring for flowing an electric current to invertsaid spontaneous magnetization so as to extend in a directionsubstantially parallel with said substrate and said first plane ofsymmetry; and arranging the center line of said wiring so as to shiftsaid center line to said first plane of symmetry.